A rebel fighter runs with a weapon on the outskirts of the northern Syrian town of al-Bab, Syria January 26, 2017.Khalil Ashawi

BEIRUT (Reuters) - Jihadists from al Qaeda's former Syria branch and rebels who have recently joined forces against them fought in heavy clashes in the northwest of the country on Friday, a rebel official and a monitoring group said.

Fighting between Jabhat Fateh al-Sham, formerly the al Qaeda-affiliated Nusra Front, and more moderate, foreign-backed factions erupted this week in areas west of Aleppo and the adjacent rebel-held province of Idlib.

The clashes, which are taking place separately to the main battle in Syria's conflict - that between rebels and the Syrian government - threaten to further weaken opposition to President Bashar al-Assad in the insurgents' biggest territorial stronghold.

A rebel source said Fateh al-Sham launched fierce new attacks on Friday.

"A short while ago there was tank bombardment of the base of the headquarters of our brothers in the Jaish al-Islam (faction) in Babsiqa," a rebel source in one of the groups involved in the fighting told Reuters.

"Activists are reporting casualties in a camp for women nearby from the tank and mortar bombardment."

The clashes appeared to take place in two areas of Idlib - one west of Aleppo and close to the Turkish border, and the other south of Idlib city, close to the main highway linking Aleppo to Damascus.

The Syrian Observatory for Human Rights monitoring group said each side was using "heavy weaponry", and reported a number of civilian casualties.

In towns close to the fighting, several hundred people protested against Fateh al-Sham for targeting rebel factions, or called for the clashes to stop so civilians would not get hurt, the British-based Observatory said.

Fateh al-Sham, which routed at least one Free Syrian Army rebel faction this week, is now fighting against a number of groups that have joined forces under the powerful Islamist Ahrar al-Sham to fend off the assault.

Ahrar al-Sham, which presents itself as a mainstream Sunni Islamist group, sided with the FSA groups and said Fateh al-Sham had rejected mediation attempts.

Fateh al-Sham said on Tuesday it had been forced to act preemptively to "thwart conspiracies" being hatched against it.

The groups it has attacked include factions that attended peace talks in Kazakhstan sponsored by Damascus allies Russia and Iran, and rebel backer Turkey.